Canadian Prairie Snowstorm of Nov 7-9, 2020

 Meteorology 

A memorable, and perhaps record-breaking winter storm is set to impact much of the prairies this weekend, bringing heavy snow and strong winds that will likely make travel difficult to impossible at times through much of southern Saskatchewan and portions of southern Alberta.

As a large upper low/trough digs into the southwest US throughout the day on Saturday, a broad deformation zone develops from the northern US Rockies through southeastern Alberta northeastwards into the central prairies. Heavy snow will develop along this feature throughout the day on Saturday, with strong northerly winds developing by late day into southern Alberta, enhancing upslope effects in the southern foothills as well as along the north slope of the Cypress Hills, resulting in blowing and drifting snow throughout much of the southern half of the province. 

As a potent shortwave trough with increasing negative tilt begins to lift northeast out of the central and northern US Rockies, a strong area of low pressure departs the mountains of northern Wyoming and reconstitutes itself northeastwards along a barolinic zone through the Dakotas and into Manitoba by late Sunday evening. As isentropic lift increases throughout the day on Sunday poleward of the surface warm frontal zone, ice pellets and freezing rain are likely along an axis stretching from eastern Montana and extreme southeastern Saskatchewan, through southwestern and central Manitoba, and into far northwestern Ontario. Meanwhile, strong cold air advection northwest of the low begins to wrap increasingly cold air into the system, with heavy snow northwest of the low’s track developing from central Montana into southern Saskatchewan, and on into central and northern Manitoba, resulting in a second round of heavy accumulations across parts of the prairies. 

As strong surface pressure rises occur northwest of the low associated with cold air spilling southwards to the east of the Rockies, strong low level winds result from southeastern Alberta through southern Saskatchewan on Sunday – and, when combined with a 50+ knot northeasterly H850 jet resulting in strong wind gusts throughout southern Saskatchewan during the latter half of Sunday as the low reaches peak intensity – whiteout conditions will result in blowing and drifting snow, making travel difficult to impossible over a broad area. 

With a broad area of 30-50cm falling across the region, it is possible that several monthly 24 hour record snow totals could be broken across portions of Saskatchewan – especially where accumulations between both rounds of snow overlap. The snow moves out of the southern prairies on Monday morning, with cold temperatures and wind chills in its wake. 

Friday 18Z NAM prog valid 6PM CST Sunday, Nov 8. 
A potent "Wyoming" low cuts through the heart of the continent.

Impacts

It is highly likely that the greatest impacts will be to travel throughout the weekend, as road closures will become likely across parts of southern Alberta and much of southern Saskatchewan between late day Saturday and early Monday. Visibilities will be reduced to near zero in blizzard conditions, with winds having a strong crosswind component throughout the Trans-Canada Highway corridor west of Regina. Snow ploughs will likely have great difficulty keeping up with clearing off  ever-building snow drifts, as large amounts of snow will be available for transport in amply strong winds. As such, all travel should be avoided across the most heavily-impacted areas of the prairies this weekend. Those who must drive should exercise extreme caution and have an emergency safety kit in their vehicle that is easily accessible.

In addition, within the aforementioned corridor of freezing rain accumulations, ice accretion will likely yield slippery road conditions and perhaps some sporadic power outages in more heavily-affected areas. 

Friday 12Z ECMWF ensemble mean snow totals as of Monday at  6PM CST.

The July 12 Tornado West of Nanton

Our view of the tornado from near Stavely.

On July 12, 2020, a tornado occurred deep in the rolling terrain of the northern Porcupine Hills about 17km WSW of Nanton on a roughly west-to-east track, between 107-120PM MDT. It impacted one property, ripping the roof off a barn, and destroying a shop - among other damage in the yard.

Meteorological Background Environment and Observations

Severe weather was anticipated in parts of southern and central Alberta on July 12. The parent tornadic supercell occurred in the southern foothills on the cyclonic shear side of a 50 knot 500mb jet streak, with continued cooling aloft during the day. The area was also beneath the left exit region of the upper jet, and north of a stalled cold front/post-frontal dryline, where low level flow had locally begun to turn upslope (southeasterly). Being early in the day, MLCAPE values were marginal (well below 1000J/kg) but ample for robust, low-topped updrafts amid 50 knots of 0-6km shear (with slightly less effective shear) - a profile that favoured highly-tilted storms, and deep updraft rotation. In many ways, the event transpired in a similar fashion to the Priddis tornado event of July 22, 2015.

We observed the tornado from the southeast, near Stavely. Immediately prior to witnessing the funnel, the RFD of the parent storm wrapped in, evaporating cloud matter and bringing about a clear slot. The tornado occurred with this first occlusion, which lasted several minutes, with the next cycle bringing about ominously intense rotation in the wall cloud - perhaps as a result of the storm interacting with the northward-surging dryline. However, the second cycle did not produce a tornado, possibly due to the cold RFD we observed, which was more established than in the first cycle. No subsequent mesocyclonic occlusions occurred, as the storm rapidly lost control of its outflow thereafter.

Modeled data (didn't grab the obs in time). Surface low in NW SK, with W-E boundary draped back across southern AB.

The storm from the SE at 107PM, at the approximate start of the tornado. The funnel is visible within the clear slot. The parent storm is a highly-tilted, low-topped supercell. 

The storm from the SE at 144PM, after which the wall cloud really began to rotate for a few minutes. 

ECCC's initial summary, which classified the tornado as a landspout (I would contest this conclusion!)

The Damage

The property owners were home at the time of the tornado, but were caught off guard at its arrival, since it had been sunny at their location immediately south of the storm. They also lacked a view up the slope to their immediate west, where the tornado entered their yard. They ran inside just in time, as the tornado destroyed a shop, before ripping the roof and gable ends entirely off a well-built barn. The barn was built in 1912, but was sturdy, and housed active horse stables. Aerial footage reveals that just a single roof panel had remained collapsed into the upper level of the barn, with the rest being scattered both aft and to the side of the tornado track. Some of the roofing debris was carried many tens of metres away from the barn. The top plate to which the roofing trusses were affixed appeared to be a double-stacked 2x6.

The barn also shifted on its foundation, with the SW corner being lifted off, and the north wall being warped and having steel anchoring material pop out of the concrete foundation. The barn is now leaning, which is apparent both outside and within the barn. Outside the barn, a portion of the roofing pile drove into the grass aft of the tornado track, becoming deeply embedded into the ground. In the shadow of the immediate downwind (east) aspect of the barn, four horses survived the ordeal, with only one of them receiving some minor scratches.

According to damage indicators, it seems that this classifies as a small barn (under 250 square metres), with a DOD of 5 (uplift or collapse of more than 50% of roof panels). Expected wind speeds associated with this damage intensity are in the neighbourhood of 150km/h (EF1), with a lower bound of 125km/h (EF0) and an upper bound of 185km/h (EF2).

Here is a link to the story on The Weather Network.